[dpdk-dev] weak functions in some drivers

["Damjan Marion (damarion)" <damarion@cisco.com>]

Hello,

We just spent few hours troubleshooting why vPMD is not working
in i40e driver. Conclusion was that problem is caused by linker 
linking the wrong instance of the i40e_rx_vec_dev_conf_condition_check(...).

That function is defined 2 times, once in i40e_rxtx.c and once in 
i40e_rxtx_vec.c. First one is defined as weak and it just returns -1.

librte_pmd_i40e.a contains both versions:

$ objdump -x librte_pmd_i40e.a| grep i40e_rx_vec_dev_conf_condition_check
0000000000006ca0  w    F .text	0000000000000006 i40e_rx_vec_dev_conf_condition_check
00000000000007c1 g     F .text.unlikely	000000000000001c i40e_rx_vec_dev_conf_condition_check

However when we are linking our app, linker was picking 1st (weak) one and vPMD init was failing.

Workaround we applied to get int working:  -Wl,--whole-archive <LIBS> -Wl,—no-whole-archive

What is not clear to me is motivation to use weak here instead of simply using CONFIG_RTE_I40E_INC_VECTOR
macro to exclude stubs in i40e_rxtx.c. It will make library smaller and avoid issues like this one
which are quite hard to troubleshoot.

BTW Looks like same issue is happening with fm10k driver.

Thanks,

Damjan